Method of avoiding strain in phase transitions of single crystals

ABSTRACT

Method for growing crystals of inorganic metal salts from a melt which comprises cooling a single crystal of an inorganic metal salt having at least one solid-solid transition point in a stable structure of a higher temperature in such a manner that the crystal passes through the transition point from an end of the crystal to another end of the same to obtain the corresponding single crystal in a stable structure of a lower temperature, the transition interface being at such a distance from the solidliquid interface that the strain caused by the solid-solid transition does not exert any influence on the solid-liquid interface.

United States Patent METHOD OF AVOIDING STRAIN IN PHASE TRANSITIONS OFSINGLE CRYSTALS 7 Claims, 6 Drawing F igs.

References Cited UNITED STATES PATENTS Emeis Primary Examiner- NormanYudkoff Assistant Examiner-R. T. Foster Att0rneyWenderoth, Lind & PonackABSTRACT: Method for growing crystals of inorganic metal salts from amelt which comprises cooling 2 single crystal of an inorganic metal salthaving at least one solid-solid transition point in a stable structureof a higher temperature in such a manner that the c rystal passesthrough the transition point US. Cl 23/296, from an end of the crystalto another end of the same to obtain 23/301, 23/97 the correspondingsingle crystal in a stable structure ofa lower Int. Cl B01j 17/04,temperature, the transition interface being at such a distance C01 g3/04 from the solid-liquid interface that the strain caused by the Fieldof Search 23/301 SP, solid-solid transition does not exert any influenceon the solid- 97,296 liquid interface.

elt

Single .L II crystal in I a stable I structure 1 of a higher II teperature 1 Single crystal in a stable structure of a. lower temp rature-PATENTEU SEF21 I971 360K137 sum 1 or 2 FIG- I INVENTORS:

T smo IHoe'ucm IcHH-HKO NlwA KATSURO NAKA zAwA ATTORNEYS PATENTEDSEP21I97! SHEET 2 OF 2 FIG. 2b

FIG. 20

emperature p mam n 33 e flst r M 1t 0 e samap ny m rn fe SCiSOtemperature p rature ingle crystal in a. stable structure of a higher teperature ingle crystal in a stable structure of a lower e p ratureINVENTORS: T'qsHlo \INOG'UCJH HlHlkO Nwm KRTSURO NhKALAWA E7 WWTL. {1

ATT RNEYS METHOD OF AVOIDING STRAKN IN PHASE TRANSITIONS OF SINGLECRYSTALS The present invention relates to a method for growing singlecrystals. More particularly, it relates to a method for growing singlecrystals of inorganic metal salts having at least one solid-solidtransition point.

Although there have been known various methods for growing singlecrystals, none of them can be applied to an inorganic metal salt thatthe transition point is comparatively contiguous to the melting point.In the case, they afford fine and polycrystalline bodies, which are notsuited to any practical use, e.g., as an electro-optical element.

As the result of various investigations, the present inventorspreviously provided a method for growing single crystals of inorganicmetal salts which comprises cooling a melt of an inorganic metal salthaving a transition point admixed with a melting point depressing agentin an amount that the melting point of the inorganic metal salt islowered below the transition point. The melting point depressing agentfor the said method should have such a small segregation constant thatit is sufficiently segregated from the mother melt on crystallization.In this respect, for instance, potassium chloride and strontium chlorideare effective melting point depressing agents for cuprous chloride.However, it is inevitable that the afforded single crystal iscontaminated with the melting point depressing agent in such a verysmall amount that the contaminant can not be detected by a usualanalytical method, even though the most suitable melting pointdepressing agent is employed. As well known, trace contaminant affordsmore or less an influence of the electric and optical characteristics asingle crystal, and such influence may be regarded as significantdepending on the use of the crystal. Thus, the said method can affordsingle crystals being large enough to be practically used but can notgive extremely pure ones.

Considering the reason for the difficulty in growing a single crystal ofan inorganic metal salt having the transition point contiguous to themelting point, the temperature gradient at the crystal-growing portionin the growing direction is considerably steep in a conventionalcrystallization method such as the so-called ascending method (e.gvCzochralski Method Kyropoulos method) or the so-called temperaturegradient method (e.g. Bridgman method, Stockbarger method), and thesolid-liquid transition interface (i.e., an isothermal interfacecorresponding to the melting point) where the inorganic metal salt iscrystallized from the melt in a stable structure of a higher temperatureis close to the solid-solid transition interface (i.e., an isothermalinterface corresponding to the transition point) where the transition ofthe crystal from the stable structure of a higher temperature to that ofa lower temperature takes place. For instance, in the case of cuprouschloride, the interval between the said two interfaces is less than 1cm., i.e., several mm. Therefore, the strain caused by the solid-solidtransition is propagated to the solid-liquid transition interface wherecrystallization is taking place, and the propagated strain results inthe occurrence of undesirable crystal nuclei which prevent the growth ofa single crystal. The said method using a melting point depressing agentovercomes this difficulty caused by the crystalline transition. As abovementioned, however, it can not avoid the contamination of the obtainedsingle crystal with a trace amount of the used melting point depressingagent.

Through the advanced investigation, it came to the notice of the presentinventors that a single crystal of an inorganic metal salt having atransition point can be readily grown at the temperature range from themelting point to the transition point in the same manner as the onelacking a transition point and also that difficulty is present in how tocool the thus-afforded crystal in a stable structure of a highertemperature to a lower temperature, e.g., room temperature. through thetransition point. Then, the polycrystallization occuring when a singlecrystal in a stable structure of a higher temperature is cooled throughthe transition point while keeping a unified temperature gradient in awhole crystal was interpreted to be caused by initiation of thesolid-solid transition at innumerable points in the crystal and growthof the crystals in a stable structure of a lower temperature from thesaid innumerable points. if this was correctly interpreted, the gradualcooling of a single crystal in a stable structure of a highertemperature through the transition point from an end of the crystal toanother end of the same should afford the corresponding single crystalin a stable structure of a lower temperature. The correctness of theinterpretation has been evidenced by the experiments, and there havebeen successfully produced single crystals of inorganic metal saltsbeing satisfactory in size and purity and suitable for electric andoptional use.

Accordingly, a basic object of the present invention is to embody asingle crystal of an inorganic metal salt having at least one transitionpoint. Another object of this invention is to embody a method forgrowing a single crystal of an inorganic metal salt of which thetransition point is contiguous to the melting point. A further object ofthe invention is to embody a method for growing a single crystal of aninorganic metal salt possessing a practically utilizable size in highpurity. These and other objects will be apparent to those conversantwith the art to which the present invention pertains from the foregoingand subsequent descriptions.

The method of this invention comprises cooling a single crystal of aninorganic metal salt having at least one solid-solid transition point ina stable structure of a higher temperature in such a manner that thecrystal passes through the transition point from an end of the crystalto another end of the same to obtain the corresponding single crystal ina stable structure of a lower temperature. The present method can beapplied equally to any inorganic metal salts so far as they have one ormore transition point(s). Examples of such inorganic metal salts arecuprous bromide, cuprous iodide, zinc sulfide, cadmium sulfide, etc. Forthe convenience of illustration, however, cuprous chloride is taken asan example in the following disclosure.

Cuprous chloride has a melting point at 422 C. and a transition point407 C. That is, the crystal is formed in the wurtzite structure at atemperature from 407 C. to 422 C. and in the zinchblende structure at atemperature lower than 407 C. Accordingly, when a melt of cuprouschloride is cooled to a room temperature in a conventional manner, thereare formed polycrystals in the zincblende structure, of which size is atthe largest about 3 mm.

To grow a single crystal of cuprous chloride in the zincblende structureaccording to the method of this invention, it is necessary first to forma single crystal in the wurtzite structure, i.e., a stable structure ofa higher temperature. For this purpose, a melt of cuprous chloride maybe cooled from an end to another end to a temperature ranging from themelting point to the transition point. Then, the crystal in the wurtzitestructure is gradually cooled so that is passes through the transitionpoint from an end to another end, whereby there is formed a singlecrystal in the zincblende structure, i.e., a stable structure of a lowertemperature. It is not necessary to initiate the transition from thewurtzite structure to the zincblende structure after completion of thecrystallization of the whole melt of cuprous chloride into the wurtzitestructure. That is, the solid-solid transition can be performedsimultaneously with the liquid-solid transition unless the strain causedby the solid-solid transition exerts influence on the liquid-solidtransition interface. For instance, an interval of more than 1 cm.between the two transition interfaces is sufficient for this purpose.

In view of the object of the present invention, the starting cuprouschloride should be sufficiently pure. For instance, it is suited to thepresent method, if neither turbidity nor coloration is observed onmelting. The purification of a commercially available cuprous chloridemay be effected in conventional procedures, e.g., by washing with asolvent such as glacial acetic acid, ethanol, acetone or the like,recrystallization from concentrated hydrochloric acid, sublimation underreduced pressure and zone melting under reduced pressure.

The crystallization may be performed in such a tube usually employed forcrystallizing a single crystal as made of quartz glass or hard glass.The glass tube that a carbon film is formed on the inner wall isadvantageous for prevention of wetting with the melt of cuprouschloride, the occasionally occasionally resulting in the occurrence ofstrain on cooling which may sometimes lead to the production of cracksand polycrystalline bodies. The formation of the carbon film may bepreferably effected by decomposing an organic compound such as methane,ethane, ether, benzene or acetone on the inner wall of the glass tubewhile heating. Although the carbon film can be also formed by any otherconventional procedure (e.g., application of colloidal black lead), suchfilm is apt to be eliminated. It is also advantageous to use the glasstube having a specifically formed bottom part for selecting a crystalnucleus so that the production of polycrystalline bodies can beinhibited. Such glass tube consists of an upper part for growing acrystal and a lower part (i.e., a bottom part) for selecting a crystalnucleus, these parts satisfying the following requirements: (a) theformer and the latter being formed as a body intervening a neck of whichthe opening has a diameter suitable for introduction of a melt andselection of a crystal nucleus; (b) the latter having at least one curveso that the former and the latter are not coaxial; and (c) the saidopening being not within the solid angle viewed from the end of thelatter along the wall of the glass tube. Some typical examples of theglass tube provided with the above requirements are disclosed in ourcopending application, Ser. No. 544,605, filed Apr. 22, 1966 now U.S.Pat. No. 3,433,602.

- The cooling manner per se may be effected according to a known method(e.g., Bridgman method, Stockbarber method).

By application of the present invention, there can be readily obtainedthe substantially unstrained and highly pure single crystal of cuprouschloride in such a size as practically utilizable (e.g., cylindricalsingle crystal of 20 mm. in diameter and 50 mm. in length).

The present invention has been hereinabove illustrated on the productionof the single crystal of cuprous chloride. How ever, it is clear thatthis invention can be generally applied for the production of singlecrystals of inorganic metal salts having a transition point in thesubstantially same manner as in the production of the single crystal ofcuprous chloride.

Practical embodiments of the present invention are illustratively shownin the following examples with reference to the attached drawings.

Example 1 A. Purification of cuprous chloride:

Commercially available cuprous chloride (reagent grade) is washed withglacial acetic acid, ethanol and ether in order in nitrogen atmosphereand dried at 75 to 100 C. in nitrogen stream. The resultant cuprouschloride is charged in a transparent quartz glass tube. After evacuationby heating at 300 C. under reduced pressure for 5 to 8 hours, the quartzglass tube is sealed and subjected to zone melting purification with azone temperature 550 to 600 C. and a rate of movement of 8 cm. per hour.The color of the melt is dark green to green until passing about 10zones and then becomes blackish brown to yellowish brown while passingfurther zones. Finally, the solid part is made colorless andtransparent. The purification is accomplished by passing about 20 zones.

B. Formation of carbon film in crystallizing tube:

An end of a transparent quartz glass pipe is sealed to form a bottompart as shown in FIG. 1, the ultimate end shaping a cone of about 60 invertical angle. The resultant tube is heated at 600 to 800 C. underreduced pressure and vaporized acetone is introduced therein. Theacetone is decomposed to form a carbon film on the inner wall of thetube.

C. Growth of single crystal:

The quartz glass tube prepared above is heated under reduced pressure toeliminate the air therein, charged with cuprous chloride, heated atabout 300 C. under reduced pressure for 5 hours and then sealed. Thequartz glass tube is suspended in a vertical furnace as shown in FIG. 2(a) having a temperature distribution as shown in FIG. 2 (b) andgradually descended. In the zone I (the temperature (T, being higherthan the melting point of cuprous chloride (MP), e.g., 450 to 500 C.),the melt of cuprous chloride is formed. Subsequently, in the zone II(the temperature (T being lower than the melting point of the cuprouschloride (mp) and higher than the transition point (TP), e.g., 410 to415 C.), a single crystal in a stable structure of a higher temperature(the wurtzite structure) grows. After the whole melt has beencrystallized into a stable structure of a higher temperature, the endofthe tube reached the zone III (the temperature (T;, being lower thanthe transition point (TP), e.g., 350 to 400 C.), where the solid-solidtransition is initiated. Finally, the whole crystal is changed to asingle crystal in a stable structure of a lower temperature (thezincblende structure). The speed of descent should be decided by thetemperature gradients of the zones 11 and III. In this example,agoodresult is achieved with a speed of about 0.5 mm. per hour, when thetemperature gradient of the zone 11 is 10 C. per cm. and that of thezone III 4 C. per cm.

Example 2 The purification of cuprous chloride, the formation of carbonfilm in crystallizing tube and the growth of single crystal are executedas in example 1 A, B and C but using a vertical furnace as shown in FIG.3 a having a temperature distribution as shown in FIG. 3 b or b.Contrary to the foregoing example 1 wherein the distance of the zone IID is longer than the length of the sample L so that the solid-solidtransition takes place after completion of the liquid-solid transition,the length of the sample L is longer than the distance of the zone 11 Din this example. Therefore, the solid-solid transition takes placebefore completion of the liquid-solid transition. In the case, thedistance of the zone 11 D is adjusted to such a distance that the straincaused by the solid-solid transition does not prevent the growth of thesingle crystal at the liquid-solid interface, i.e., more than 1 cm.

What is claimed is:

l. A method for growing a single crystal of an inorganic metal salthaving a solid-solid transition point contiguous with its melting pointwhich comprises cooling a single crystal of the inorganic metal saltfrom a melt, the single crystal forming at a solid-liquid interface, thecooling being in such a manner that the crystal passes through thetransition point from one end of the crystal to the other whereby thesingle crystal is converted from the form stable above the transitionpoint to the form stable below the transition point, the temperaturegradient on cooling being so gradual and the solid-liquid interfacebeing at such a distance from the solid-solid transition interface thatthe strain caused by the solid-solid transition does not exert anyinfluence on the liquid-solid interface.

2. The method according to claim 1, wherein the single crystal passesthrough the transition point after its crystallization from thecorresponding melt is completed.

3. The method according to claim 1, wherein one portion of the singlecrystal passes through the transition point while another portion isbeing crystallized from a melt.

4. The method according to claim 1, wherein the cooling is effected in aquartz glass tube provided with a carbon film on the inner wall.

5. The method according to claim 1, wherein the inorganic metal salt iscuprous chloride.

6. The method according to claim 1 wherein, the distance between thesolid-liquid interface and the solid-solid transition interface is morethan 1 cm.

7. A method according to claim 1, wherein the final single crystal isobtained by first passing the inorganic metal salt ture lower than thesolid to solid transition temperature of the inorganic metal saltwhereby a single crystal having a structure stable at a lowertemperature is obtained, the distance between the solid-liquid interfaceand the solid-solid transition interface being more than 1- cm.

2. The method according to claim 1, wherein the single crystal passesthrough the transition point after its crystallization from thecorresponding melt is completed.
 3. The method according to claim 1,wherein one portion of the single crystal passes through the transitionpoint while another portion is being crystallized from a melt.
 4. Themethod according to claim 1, wherein the cooling is effected in a quartzglass tube provided with a carbon film on the inner wall.
 5. The methodaccording to claim 1, wherein the inorganic metal salt is cuprouschloride.
 6. The method according to claim 1 wherein, the distancebetween the solid-liquid interface and the solid-solid transitioninterface is more than 1 cm.
 7. A method according to claim 1, whereinthe final single crystal is obtained by first passing the inorganicmetal salt through one heat zone at a temperature above the meltingpoint of the inorganic metal salt and gradually passing the melt througha second heat zone at a temperature lower than the melting temperatureof the inorganic salt but above the transition point of the inorganicmetal salt to form a single crystal of a stable structure at a highertemperature and then gradually passing the crystal through a heat zonewhich has a temperature lower than the solid to solid transitiontemperature of the inorganic metal salt whereby a single crystal havinga structure stable at a lower temperature is obtained, the distancebetween the solid-liquid interface and the solid-solid transitioninterface being more than 1 cm.